Recently there has been renewed interest in “innovative” or “functional” dyes. One area of interest is that of optical recording technology where gallium aluminium arsenide (GaAlAs) and indium phosphide (InP) diode lasers are widely used as a light source. Since dyes absorbing in the near infrared (near-IR) region (i.e., beyond about 700 nanometers in wavelength and less than about 2000 nanometers in wavelength) are required and the oscillation wavelengths fall in the near-infrared region, they are suitable candidates for use as infrared dyes.
Infrared dyes have applications in many areas. For example, infrared dyes are important in the optical data storage field, particular in the DRAW (Direct Reading After Writing) and WORM (Write Once, Read Many) disk, which is used for recording. Currently, indolinocyanine dyes, triphenylmethane dyes, naphthalocyanine dyes and indonanaphthalo-metal complex dyes are commercially available for use as organic colorants in DRAW disks. Cyanine dyes can only be used if additives improve the lightfastness.
Another application of infrared dyes is in thermal writing displays. In this application, heat is provided by a laser beam or heat impulse current. The most common type of infrared dyes used in this application are the cyanine dyes, which are known as laser dyes for infrared lasing.
Infrared dyes are also used as photoreceptors in laser printing. Some infrared-absorbing dyes are used in laser filters. They also find application in infrared photography and even have application in medicine, for example, in photodynamic therapy.
The compounds of the present invention will now be described in the context of printing inks and the like, but it will be understood by the skilled reader that the compounds described hereunder may be used in other applications, for example, those set out above.
Fast, error-free data entry is important in current communication technology. Automatic reading of digital information in printed, digital and analog form is particularly important. An example of this technology is the use of printed bar codes that are scannable. In many applications of this technology, the bar codes are printed with an inks that are visible to the unaided eye. There are, however, applications (eg security coding) that require the barcode or other intelligible marking to be printed with an ink that invisible to the unaided eye but which can be detected under UV light or infrared light (IR).
For instance, U.S. Pat. No. 5,093,147 describes a method exploiting the process of fluorescence in which a dye is excited by ultra-violet (UV), visible or near-IR radiation and fluorescent light emitted by the dye material is detected. This reference describes a jet printing process used to apply a compatible liquid or viscous substance containing an organic laser dye that is poorly absorptive of radiation in the visible wavelength range of about 400 nm to about 700 nm, and is highly absorptive of radiation in the near-IR wavelength range of about 750 nm to about 900 nm. The dye fluoresces at longer wavelengths in the IR in response to radiation excitation in the near-IR range.
Another example is described in U.S. Pat No. 5,460,646 (Lazzouni et al) which describes the use of a colorant which is silicon (IV) 2,3-naphthalocyanine bis((R1)(R2)(R3)-silyloxide) wherein R1, R2, and R3 are selected from the group consisting of an alkyl group, at least one aliphatic cyclic ring, and at least one aromatic ring.
The infrared absorbing dyes Squarylium and Croconium dyes have been extensively described in the literature (see for example, T. P. Simard, J. H. Yu, J. M. Zebrowski-Young, N. F. Haley and M. R. Detty, J. Org. Chem. 65 2236 (2000), and J. Fabian, Chem. Rev. 92 1197 (1992)). These prior art compounds have a central squarylium or croconium moiety connected to traditional electron donors. These donors act to donate an electron to the central squarylium or croconium moieties. However, these particular dyes do not absorb at a high enough wavelength and/or also absorb too strongly in the visible spectrum. Secondly, it is the infrared absorbing property of the molecule when it is not in solution that is more important in this particular application than when it is solvated. That is, whether the ratio of infrared absorption to visible absorption of the colorant on a surface is still acceptable for use as a security ink or like applications. This was brought into question by such groups as D. Keil, H. Hartmann and C. Reichardt, Leibigs Ann. Chem. 935 (1993). For example, they showed that when a croconate dye was deposited onto a polymer surface that the sharp infrared absorption peak becomes a very broad peak that contains a large shoulder in the visible part of the spectrum. This may be explained by the lack of rigidity of the molecule, which may be maintained while in a solvent, and/or that intermolecular interactions with other molecules, while not in a solvent, causes a hypsochromic shift of the absorption peak. A molecular dynamics simulation of a typical squarylium dye shows that the infrared absorption peak becomes spread out so much so that the ratio of infrared to visible absorption decreases by about two orders of magnitude. Hence the rigidity of conventional infrared dyes needs to be addressed and/or a possible method is used to decrease intermolecular interactions when they are deposited onto a surface.